Abstract. Methane is a significant atmospheric trace gas in the context of greenhouse
warming and climate change. The dominant sink of atmospheric methane is the
hydroxyl radical (OH). Recently, a mechanism for production of chlorine
radicals (Cl) in the marine boundary layer (MBL) via bromine autocatalysis
has been proposed. The importance of this mechanism in producing a methane
sink is not clear at present because of the difficulty of in-situ direct
measurement of Cl. However, the large kinetic isotope effect of Cl compared
with OH produces a large fractionation of 13C compared with 12C in
atmospheric methane. This property can be used to estimate the likely
minimum size of the methane sink attributable to MBL Cl. By taking account
of the mixing of MBL air into the free troposphere, we estimate that the
global methane sink due to reaction with Cl atoms in the MBL could be as
large as 19Tgyr-1, or about 3.3% of the total CH4 sink.
However, its impact on the methane stable carbon isotope budget is large and
warrants further attention.